Chemical sensors are used in various applications including environmental emission control, agri-food industry, and other industrial applications. They are also used in biomedical applications and clinical analysis for determining the pH, the presence of specific ions or the oxygen or alcohol concentration in a sample solution for example. Optical chemical analysis methods include, for example, Fourier transform infrared spectroscopy.
Optical fiber based optodes generally use a sensing membrane deposited at the fiber tip which was previously cleaved and polished. Alternatively, evanescent wave spectroscopy uses an optical waveguide that is immersed into the sample solution. Light is guided in the waveguide by internal reflection at the waveguide-solution interface. The refractive index of the waveguide is higher than that of the solution so that the solution acts as a cladding for the optical waveguide. Light is mostly propagated in the waveguide but part of the light, namely the evanescent wave, propagates in the solution (acting in a way similar to a waveguide cladding) and can then be absorbed by the analyte. Analysis of the measured absorption spectra provides an indication of the presence of given chemicals.
The use of a polymer membrane cladding on an optical waveguide was also proposed as an alternative design of the evanescent wave sensor. In this case, the analyte diffuses in the membrane when the optical waveguide is immersed in the sample solution. As light is guided by the core, the evanescent wave propagates in the polymer membrane and the optical absorbance of the analyte which diffused in the cladding is measured.
In conventional multimode optical waveguides, distribution of the optical power between the core and the cladding is different for each mode. Low-order modes are much confined in the core of the fiber compared to high-order modes, the latter interacting more strongly with the cladding or surrounding sample solution. The high-order modes are then depleted rapidly by evanescent wave absorption compared to the low-order modes. The waveguide optical absorbance is therefore not proportional to that of the cladding, the absorbance being defined as minus the logarithm of the light transmittance of a material or a device. Accordingly, in Payne, F. P. and Z. M. Hale, “Deviation from Beer's law in multimode optical fiber evanescent field sensors.”, International Journal of Optoelectronics, 8, 743, 1993, it was demonstrated that the Beer-Lambert law, which linearly relates the absorbance of an optical waveguide to the concentration of an absorbing species in the cladding or in a surrounding solution is inapplicable in the case of evanescent wave spectroscopy with multimode fibers. Reliable quantification of the concentration of the absorbing species is therefore not straightforward. It is noted that the Beer-Lambert law usually applies to light propagating through a flat medium and says that the absorbance of the medium is proportional to the concentration of the absorbing chemicals it contains and to the light propagation length in the medium.